The conversion of solid carbon sources to carbon monoxide has been under development for many years. Such processes are of great interest since they generally have the capability of providing an environmentally clean process for carbon utilization, of producing a high heating value gas which can substitute for natural gas, and of producing a low heating value gas suitable for use as synthesis gas for subsequent conversion to hydrocarbons or chemicals or as boiler fuel.
However, many of such processes introduce air into the reactor, which is generally undesirable since large volumes of nitrogen must be handled in the process. These nitrogen gases have no function in a coal gasification process and add to the process costs as well as to the equipment size. To avoid the use of air, a system has been proposed wherein zinc oxide is used as the oxygen source. While such a process has many advantages over the use of air, it has the problem of loss of zinc values in the ash byproduct. Such values are generally in the form of zinc sulfide.